This invention relates to a paint facility. For example, a paint facility for painting truck cabs, parts, and accessories.
Four major criteria were considered in developing this paint facility (1) quality of the painted surface, (2) air pollution control, (3) energy conservation, and (4) paint transfer efficiency. The stated order of these four criteria does not reflect their priority or importance. However, the need to meet emissions limitations was the primary consideration and satisfying this need in an energy-efficient system was a synergistic result. Reference is hereby made to an article entitled, "Designing Conservation into Air Pollution Control Systems" by W. E. Harrison. The subject matter of this article is incorporated herein by reference and made a part hereof
A clean environment is necessary in a paint booth to obtain a high-quality finish It is conventional to filter the air coming into a paint booth and for workers within the booth to wear overclothes made of non-fiber-shedding clothing. It is also conventional to clean the objects to be painted prior to bringing them into the paint booth, and to take steps to insure that the compressed air and the paint are not contaminated. Despite these precautions, paint flaws have not been decreased to an acceptable level. Correction of flawed surfaces is possible; however, it is a time-consuming and expensive process. Furthermore, sanding down flaws produces dust which must be eliminated prior to repainting or the problem will be compounded It has been established by careful examination of paint flaws that the dust and fibrous particles that have caused flaws of a size visible to the naked eye have not passed through the filter, or broken off from the fibrous material of the filter used to clean the incoming air. The more likely source for such dust and fibrous particles is that they were carried into the paint booth by paint booth personnel or entered through the access area through which the workers enter and leave the paint booth. Reference is hereby made to an article by Mr. Carl Frudenberg entitled, "Dust-Free Painting". The subject matter of this article is incorporated herein by reference and made a part hereof. By utilizing computer-controlled robots to apply the paint, people carrying dust on their clothing have been eliminated from the spray booths which was a significant source of flaw causing material. Probably the most significant aspect of this phenomenon is the virtual elimination of the need to open the paint booth to the external environment to permit the entrance and egression of the paint booth workers
The transfer efficiency of a spray painting device is the percent of paint leaving the nozzle that is transferred to the object to be painted. As the transfer efficiency is increased the amount of wasted paint decreases and there is a corresponding decrease in the volatile organic compounds (VOC's) deposited into the spray booth atmosphere. If 100% transfer efficiency is attained the VOC's deposited into the spray booth atmosphere will be minimized. Electronic robot and electrostatic spray guns greatly increase transfer efficiency by creating an electrical attraction between the paint and the object to be painted. When electrostatic spraying, electrically charged paint droplets move along lines of force which exist between the electrically charged spray gun and the grounded part to be painted. Reference may be had to U.S. Pat. No. 4,679,734 for a disclosure of an electrostatic spray gun. The subject matter of this patent is incorporated herein by reference.
The utilization of robots to apply paint further improves the transfer efficiency of a painting process. Optimal paint patterns for the robot, that minimize paint missing the product to be painted and applying thicker coats then desired, can be developed and repeated for each product.
Since this is a continuous painting process both the entrance and the exit to the paint booth must be open to permit the articles to be painted into and out of the paint booth. The atmosphere in the spray booths includes a level of volatile organic compounds (VOC) that for health, safety and environmental reasons cannot be permitted to escape into the room that houses the paint booth. This is accomplished by introducing air under pressure, directed toward the first spray booth, into the tack off area of the paint booth and removing an equal amount of air from the prime repair booth. The air flow through each spray booth is downwardly pulling with it some atmosphere from the adjacent flash off tunnels. Air is also introduced into the exit end of the third flash off tunnel and is directed back toward the third spray booth. In this way the high concentration of VOC's flowing downwardly through the spray booths is sealed within the paint booth by the air flow toward the first and third spray booths.
Another type of paint flaw is called "sag". This flaw is caused if the newly applied paint flows or sags, forming a build-up or thickened area of paint. Sagging occurs if the paint solvents do not evaporate or "flashoff" fast enough, thus permitting the wet paint to flow or sag. The occurrence of this flaw is influenced by the percent saturation of solvent in the paint booth atmosphere. When the percent saturation of solvent in the atmosphere is high, the rate of evaporation or flash-off of the solvent of newly applied paint is slowed and the newly applied paint will flow or sag. Thus, it is important to control the percent saturation of solvent in the paint booth atmosphere, maintaining it at levels that will not adversely affect the flash-off rate of newly applied paint. It has been found that the solvent concentration of one solvent in the paint booth atmosphere does not affect the evaporation or flash-off of a second different solvent. Thus, by constructing the paint to include a plurality of solvents, the solvent concentration of each of the several solvents can be maintained at low levels. It has been found that percent saturation of a solvent begins to hinder solvent evaporation time before reaching significant lower explosive limit risk levels for volatile organic compounds. Utilizing a paint having at least three different solvents, none of which comprise more than 50 percent of the total solvents, has been found to be a solution to the sag flaw problem. It has been found that an upper limit of fifteen (15) percent saturation for any one solvent provides a limit for paint quantity environment. The use of high vapor pressure solvents such as Methyl Ethyl Ketone results in quick flashoff as compared to solvents, such as Methyl Amyl Ketone, having low vapor pressures and corresponding relatively slow flashoff. Also it is well known in the paint industry to use sag control agents in mixing paints.
When a liquid evaporates into a limited space, two opposing processes are in operation. The process of vaporization tending to change the liquid to the gaseous state and the process of condensation which tends to change the gases back into the liquid state. The rate of condensation is increased as vaporization proceeds and the pressure of this vapor increases. If sufficient liquid is present, the pressure of the vapor ultimately reaches a value at which the rate of condensation equals the rate of vaporization. When this condition is reached, a dynamic equilibrium is established and the pressure of the vapor will remain unchanged. The pressure exerted by the vapor at such equilibrium conditions is termed the vapor pressure of the liquid. The vapor pressure of a liquid is determined by its intermolecular attractive forces. Thus, if a substance has relatively low intermolecular attractive forces, the rate of loss of molecules from its surface is large and the corresponding equilibrium vapor pressure is high. Solvents such as Xylene, Butanol, Methyl Amyl Ketone, P.M. Acetate, Ektasolve EEP and Butyl Cellosolve Acetate have relatively high intermolecular attractive forces, their rate of vaporization is low, and their vapor pressure is low.
The following table provides the Evaporation Rates and Vapor Pressures of a number of common solvents. Solvents having a Relative Evaporation Rate greater than 1.0 are classified as High Vapor Pressure Solvents while those having a Relative Evaporation Rate less than 1.0 are classified as low vapor pressure solvents. It should be noted that there is a general direct relationship between Relative Evaporation Rate and Vapor Pressure, i.e. a solvent having a high Relative Evaporation Rate has a high Vapor Pressure. However, there are some exceptions to this generalization. For example, Butyl Acetate--Xylene and Methyl Amyl Ketone--P.M. Acetate. Although the quick flashoff of high vapor pressure solvents is desirable, such solvents tend to exceed our upper limit of 15% saturation. As a result solvents classified as low vapor pressure solvents are used in formulating paint for use in this process.
______________________________________ Vapor Pressure Relative Evaporation (Millimeters Rate of Mercury at (Butyl Acetate = 1.0) 20.degree. C.) ______________________________________ Ethyl Acetate 6.2 76 Methyl Ethyl Ketone 5.7 70 Isopropanol 1.7 31 Toluene 1.5 38 Isobutyl Acetate 1.45 13 Butyl Acetate 1.0 8 Xylene 0.75 9.5 Butanol 0.45 4 Methyl Amyl Ketone 0.4 2.1 P.M. Acetate 0.34 3.8 Ektasolve EEP 0.12 1.5* Butyl Cellosolve Ace- 0.03 0.29 tate ______________________________________ *25.degree. C.
If a paint booth is 100 percent or nearly so vented to the atmosphere, and the exhausted atmosphere is replenished by fresh air, the volatile organic compounds (VOC) released to the atmosphere are uncontrolled and likely to be unacceptable, depending upon their concentrations in the paint as applied and could be considered a violation of current Environmental Protection Agency (EPA) regulations. In a conventional paint booth employing human workers, the VOC level must, to comply with OSHA rules, be maintained at a relatively low level. In order to meet EPA regulations the VOC's must be removed or reduced considerably from the paint line atmosphere prior to releasing it to the outside atmosphere. Such spray booth exhaust is a major source of VOC emissions from a paint system since 90 percent of the total VOC's is volatized in the spray booth and flash-off areas.
The atmosphere within the paint booth also becomes contaminated with paint particulate. A scrubber located at the bottom of the paint booth is designed to reduce to 11/2 to 2 grains of paint particulate per 1000 standard cubic feet per minute of exhaust air. The scrubber consists of a pair of flood sheets extending the length of the paint booth. The lower longitudinal edges of the flood sheets are spaced from each other and form a venturi. Water-including detackification chemicals flow down the flood plates and through the venturi. This low energy venturi creates 6" to 6.5" water static pressure drop. The spray booth atmosphere also flows through this venturi and is exposed to the low pressure causing the paint particles to drop out of the atmosphere. Below the venturi the paint particulate is collected in a water collecting sump formed in the bottom of the paint booth, and the atmosphere flows through a mist eliminator to the discharge duct. Paint sludge forms and floats on the surface of this water. Very low levels of solvent are found in the water and sludge (2-3% in the water and 1% in the sludge). The moisture concentration of the atmosphere flowing out the discharge duct is near saturation (95 to 100 percent relative humidity).
In a spray booth in which there are no human workers a much higher level of VOC can be tolerated. By monitoring the VOC levels and insuring the levels do not exceed 25 percent of the lower explosive level (LEL) a higher then typically experienced yet safe environment has been attained. By increasing the VOC levels of concentration it is possible to optimize the performance of a thermal oxidizer system in which the VOC's are oxidized, and thus, not released to the outside atmosphere. The heat generated from oxidation of the VOC's is used to heat the paint booth exhaust atmosphere to a temperature at which little or no extra fuel is needed in the oxidation process. The thermal oxidizer operating cost will be reduced significantly if the solvent concentration in the paint booth exhaust is high enough to support combustion without auxiliary fuel. Theoretically, operating at 3 percent of the LEL will result in autogenous combustion. The exhaust from the thermal oxidizer is relatively clean and can be released to the outside atmosphere.
The recirculated atmosphere, as well as the make-up air, is filtered and conditioned before it is directed into the top of the paint booth. Proper operation of the paint booth requires a constant booth environment in the range of 65-80.degree. F. and 55-85% relative humidity. Precise control of the spray booth temperature and relative humidity greatly enhances the quality of the painted product since it provides a constant year-round standard painting environment thus eliminating the need to adjust the paint formulations to compensate for seasonal weather variations as is required in conventional paint line applications. Filtering and conditioning of the recirculated atmosphere occurs in what is called the air house. In the production facility utilizing this system a nominal temperature of 70.degree. F. and nominal relative humidity of 70% are maintained.